The efficiencies of a number of electrochemical devices (e.g., fuel cells and metal-air batteries) are mainly governed by the kinetics of the oxygen reduction reaction (ORR). Among all the good ORR catalysts, the partially substituted double perovskite oxide (AA'B2O5+delta) has the unique layered structure, providing a great flexibility regarding the optimization of its electronic structures and physicochemical properties. Here, it is demonstrated that the double perovskite oxide, i.e., NdBa0.75Ca0.25Co1.5Fe0.5O5+delta, is a good ORR catalyst at both room and elevated temperatures. Under ambient condition, its half-wave potential of ORR in alkaline media is as low as 0.74 V versus RHE; at 650 degrees C, the cathodic polarization resistance is merely 0.0276 Omega cm(2) according to a symmetric cell measurement, whereas the solid oxide fuel cells using this cathode exhibit a maximum power density of 1982 mW cm(-2). From various materials characterizations, it is hypothesized that its excellent ORR activity is strongly correlated with the crystallographic, electronic, and defect structures of the materials.